In order to clean a solid surface so that such surface can again be coated such as, for example, to preserve metal against deterioration, remove graffiti from stone or simply to degrease or remove dirt from a solid surface, it has become common practice to use an abrasive blasting technique wherein abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminants. Various abrasive blasting techniques have been utilized to remove coatings, grease and the like from solid surfaces. Thus, blasting techniques comprising dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 3,000 psi and above, multi-step processes comprising dry or wet blasting and a mechanical technique such as sanding, chipping, etc. and a single step process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. Pat. No. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage have been used. Water for dust control has been mixed with the air either internally in the blast nozzle or external of the nozzle at the targeted surface to be cleaned and such latter process, although primarily a dry blasting technique, is considered wet blasting inasmuch as media recovery and clean up is substantially different from that utilized in a purely dry blasting operation.
The blast media or abrasive particles most widely used for blasting surfaces to remove adherent material therefrom is sand. Sand is a hard abrasive which is very useful in removing adherent materials such as paint, scale and other materials from metal surfaces such as steel. While sand is a most useful abrasive for each type of blasting technique, there are disadvantages in using sand as a blast media. For one, sand, i.e., silica, is friable and upon hitting a metal surface will break into minute particles which are small enough to enter the lungs. These minute silica particles pose a substantial health hazard. Additionally, much effort is needed to remove the sand from the surrounding area after completion of blasting. Still another disadvantage is the hardness of sand itself. Thus, sand cannot readily be used as an abrasive to remove coatings from relatively soft metals such as aluminum or any other soft substrate such as plastic, plastic composite structures, concrete or wood, as such relatively soft substrates can be excessively damaged by the abrasiveness of sand. Moreover, sand cannot be used around moving parts of machinery inasmuch as the sand particles can enter bearing surfaces and the like.
An alternative to sand as a blast media, particularly, for removing adherent coatings from relatively soft substrates such as softer metals as aluminum, composite surfaces, plastics, ceramic tile, concrete and the like is sodium bicarbonate. While sodium bicarbonate is softer than sand, it is sufficiently hard to remove coatings from aluminum surfaces and as well remove other coatings including paint, dirt, and grease from non-metallic surfaces without harming the substrate surface. Sodium bicarbonate is not harmful to the environment and is most advantageously water soluble such that the particles which remain subsequent to blasting can be simply washed away without yielding environmental harm. Since sodium bicarbonate is water soluble and is benign to the environment, this particular blast media has found increasing use in cleaning dirt, grease and oil and the like from hard surfaces such as steel as well as interior surfaces such as those which contact food such as in environments of food processing or handling.
Sodium bicarbonate is also a friable abrasive and, like sand, will form a considerable amount of dust during the blast cleaning process. To control the dust formed by the sodium bicarbonate blast media as it contacts the targeted surface, water has been included in the pressurized fluid carrier medium. Thus, water has been used as the primary carrier fluid or, more preferably, injected into a pressurized air stream which carries the blast media from the blast nozzle to the targeted surface. Water as a means to control dust has been mixed with the air stream internally in the blast nozzle or into the air stream externally of the nozzle. The addition of water to the pressurized air stream has been very effective in controlling dust formed by the sodium bicarbonate blast media. However, the addition of water can reduce productivity and, thus, purely dry blast cleaning processes are widely used.
Water has also been used as a dust control agent utilizing sand as the blast media. Moreover, most of the blast media which is utilized including the harder abrasives such as sand or alumina and the softer abrasives including the water soluble blast media such as sodium bicarbonate, sodium sulfate, sodium silicate and water insoluble soft media such as plastics, corn cobs, rice hulls, walnut shells, etc., are friable and will form a considerable amount of dust as the blast media contacts the targeted surface and strips the adherent contaminants therefrom. In an outdoor environment, the use of water as the carrier fluid or the addition of water to the pressurized air stream is useful in removing the dust formed during blast cleaning from the immediate environment of the substrate and the operator who is performing the blasting. Any dust which is not contained and removed disperses throughout the environment.
The use of blast chambers to strip contaminants from the surfaces of articles, in particular, articles which are of relatively small size is an important technique of cleaning such articles especially if blast cleaning is done on a regular basis. For example, automobile manufacturers use blast chambers to clean auto parts such as door panels, hoods, trunk tops, bumpers, etc. which have been over or unevenly painted for repainting. Blast equipment manufacturers and blast media formulators constantly test products in blast chambers. Thus, if blast cleaning is done on a regular basis, it is important to control noise levels, capture, examine and, if possible, recycle used abrasive media and control the dust which is formed so as to provide for the visibility of the blast nozzle operator. Dust control in blast chambers is extremely important in view of the enclosed environment which is used for the cleaning process. In the more enclosed environment of a blast chamber, the use of a secondary water stream does not effectively control the dust and, may in fact, add to visibility problems by forming a mist which stagnates in the chamber. Additionally, as before-said, the use of a secondary water stream for dust control may reduce the productivity of stripping and, thus, it may be desired to blast clean using a purely dry blast cleaning process. In a dry blast cleaning process, a considerably large amount of dust engulfs the environment of the target substrate. Accordingly, there is a continuing need to contain dust from blast cleaning operations, in particular, when such blast cleaning is done within a blast chamber.
It is well known to capture paint overspray whether as a liquid or as a powder by use of water curtains which are placed behind the substrate being painted. The curtains are provided by directing water downward on a flat support to form a coherent sheet of water which catches the paint particles or droplets. In blast cleaning operations, however, the abrasive blast media particles are orders of magnitude more massive than paint and the pressurized fluid which directs the blast media to the substrate is provided at a substantially greater volume than that used for paint spraying. Presently, many painting procedures even use airless systems. Consequently, the momentum of the blast media/air mixture is orders of magnitude greater than that found in painting systems such that mere water curtains are not sufficient to capture much of the deflected media and dust formed therefrom. Blast media effluent would literally pass through the water curtain and deflect off the support surface without being captured.
Accordingly, it is the primary objective of the present invention to provide a means for controlling the dust which is formed by blast cleaning a substrate in a relatively enclosed environment.
Another object of the present invention is to provide a means for dust containment and capture for use in an enclosed blast chamber which is relatively easy to manufacture, install and use.
Still another object of the present invention is to provide an improved process for blast cleaning articles within the confines of a blast chamber and whereby the dust formed during the blast cleaning process is readily removed from the environment of the operator and workpiece and eventually removed from the interior of the blast chamber.